17-aF $_{{rms}}$ Resolution Noise-Immune Fingerprint Scanning Analog Front-End for Under-Glass Mutual-Capacitive Fingerprint Sensors

Abstract

This paper proposes a fingerprint scanning analog front-end (AFE) for a 41 x 32 under-glass mutual-capacitive fingerprint sensor. As the mutual-capacitive fingerprint sensor is a smaller version of a projected-capacitive touch screen, this transparent fingerprint sensor can be mounted underneath screen cover glass. However, such glass significantly diminishes the signal-to-noise ratio (SNR) of fingerprint scanning AFE in proportion with increasing glass thickness. Moreover, external noise interferences from the display and charger are severe in displays with thin form factor. The proposed fingerprint scanning AFE can achieve a 17 atto-farad capacitance resolution using high-voltage (20 V) transmitters and multi-channel receivers comprising a pipelined readout amplifier, mixer, and second-order low-pass filter (128 kHz). A differential sensing structure and band-pass filtering are employed in the receiver front-end to enhance the noise immunity. A differential phase-encoded sequential driving transmitter with a proposed on-chip replica channel mitigate random offsets in the readout amplifier with high matching accuracy. Measurement results show that the fingerprint scanning integrated circuit (IC) fabricated by a 0.18 mu m BCD (Bipolar-CMOS-DMOS) process achieved a 13.4 dB SNR at a frame rate of 120 Hz under a 0.2 mm-thick cover glass. The prototype IC provides 20 V $_{{PP}}$ noise immunity from 0 to 500 kHz and consumes 23.2 mW from a 3.3 V supply.